docs: def add_missing_placeholders(self):
          """
          Add missing placeholders from templates. Return `True` if any missing
          placeholders were created.
          """
          content_type = ContentType.objects.get_for_model(self)
          result = False
          if self.layout:
              for data in self.layout.get_placeholder_data():
                  placeholder, created = Placeholder.objects.update_or_create(
                      parent_type=content_type,
                      parent_id=self.pk,
                      slot=data.slot,
                      defaults=dict(
                          role=data.role,
                          title=data.title,
                      ))
                  result = result or created
          return result